Magnetic bubble memories are now well known in the art. One mode of moving bubbles employs electrical conductors and is described in A. H. Bobeck et al, U.S. Pat. No. 3,460,116 issued Aug. 5, 1969. Another mode for moving magnetic bubbles is known as the "field-access" mode. This mode employs a patten of elements, typically magnetically soft permalloy, responsive to a magnetic field reorienting in the plane of bubble movement for generating changing patterns of field gradients to move bubbles. The field-access mode is disclosed in A. H. Bobeck, U.S. Pat. No. 3,534,347, issued Oct. 13, 1970.
Recently, a new type of bubble memory has been disclosed in which an apertured electrically-conducting layer is adapted for substantially uniform overall current flow thereacross. The apertures cause localized perturbation in the current flow resulting in a pattern of field gradients for moving bubbles. The apertures are arranged in positions offset from rest positions to which bubbles move when the current-produced field gradients cease. Thus, a two-phase operation results in unidirectional bubble movement. Bubble memories of this type are disclosed in A. H. Bobeck, patent applications Ser. Nos. 857,921 and 857,925, filed Dec. 6, 1977.
Memories of this recent type are characterized by two problems. One is the alignment of the rest positions and the apertures in the conducting layer. The rest positions are typically defined by ion-implantation and the resulting regions have to be aligned to within one-eighth period of the propagation pattern. For present industrially available photolithographic techniques, one micron line widths are achievable leading to propagate periods of eight microns. Smaller periods, of course, are desirable.
Another problem with these memories is that movement of a bubble to a rest position occurs at a slower speed than movement from a rest position. The reason for this is that one can overdrive with high amplitude drive pulses to move a bubble from a rest position. But there is a trade-off in the movement to the rest position. For example, if the energy difference between the offset position and the rest position is too high, the drive pulses have to be increased to compensate. This leads to higher power requirements. On the other hand, if the energy difference is low, the drive on the bubble causes relatively slow movement to the rest position.